Well, my one and only girlfriend so far (now ex) wore her hair medium length for quite a while. Eventually though, she got sick of styling it, and cut it very short. I didn't really like the way she looked with it, but it made her happy, so I kept quiet.
Personally, I'd prefer shoulder-length hair or so, but not much longer than that. One of the women in my family even grew her hair down past her backside at one point (no joke).